The present invention relates to a semiconductor device, and more specifically, to a method of fabricating nonvolatile memory capable of storing multi-bits binary information and the novel device structure.
The semiconductor industry has been advanced to the field of Ultra Large Scale Integrated Circuit (ULSI) technologies. The fabrication of the nonvolatile memories also follows the trend of the reduction in the size of a device. The nonvolatile memories include various types of devices. Different types of devices have been developed for specific applications"" requirements in each of these segments. Flash memory is one of the segments of nonvolatile memory devices. The device includes a floating gate to storage charges and an element for electrically placing charge in and removing the charges from the floating gate. One of the applications of flash memory is BIOS for computers. Typically, the high-density nonvolatile memories can be applied as the mass storage of portable handy terminals, solid-state camera and PC cards. It is because that the nonvolatile memories exhibit many advantages, such as memory retention without power, fast access time, low power dissipation in operation, and robustness.
The formation of nonvolatile memories toward the trends of low supply power and fast access, because these requirements are necessary for the application of the mobile computing system. Flash memory needs the charges to be hold in the floating gate for a long period of time. Therefore, the dielectric that is used for insulating the floating gate needs to be high quality in insulation and good durability in writing. At present, the low voltage flash memory is applied with a voltage of about 5V to 10V during charging or discharging the floating gate. As known in the art, the tunneling effect is a basic technology in charging or discharging. In order to attain high tunneling efficiency, the thickness of the dielectric between the floating gate and substrate have to be scaled down due to the supply voltage is reduced. The data program method of a non-volatile memory device includes a method using Fowler-Nordheim (FN) tunneling or a method using hot electron injection. In FN tunneling, a high voltage is applied to a control gate to induce a high electric field in a tunnel oxide layer, and electrons of a semiconductor substrate pass the tunnel oxide layer and are injected into a floating gate. During the mode of erasing, the bias may apply on the source to discharge the electron from the floating gate to the source of a memory device.
Conventional memory cell consists of a source region, a drain region, a floating gate (FG), a control gate (CG) and insulation films. A plurality of sectors are arranged in the two-dimensional manner on a semiconductor substrate of the flash memory. Memory cells are separated from one another by an element isolation region of LOCOS (local oxidization of silicon) or STI (shallow trench isolation). When writing a new data or rewriting a data stored in the flash memory, the stored data in memory cells are erased on the sector basis immediately before the writing. The device is called flash due to the data is erased sector by sector.
The prior art single bit charge trapping dielectric flash EEPROM memory cells are constructed with a charge trapping ONO layer. The memory cell generally includes a P-type silicon substrate and two PN junctions between N+ source and drain regions and P type substrate, a nitride layer sandwiched between two oxide layers and a polycrystalline layer. To program or write the cell, voltages are applied to the drain and the gate and the source is grounded. These voltages generate a vertical and lateral electric field along the length of the channel from the source to the drain. This electric field causes electrons to be drawn off the source and begin accelerating towards the drain. The hot electrons are generated at the boundary between drain and channel during the acceleration. These hot electrons are then redirected vertically into the ONO layer. Two of the ONO memories capable for storing two bits binary can refer to U.S. Pat. No. 6,011,725 to Boaz and U.S. Pat. No. 6,335,554 to Yoshikawa.
To further understand the role of xe2x80x9cfloating gatexe2x80x9d, please refer to FIG. 1A, it illustrates the well-known programming of the flash device. During the mode of programming, positive bias is applied on the control gate 105 for tunneling the carriers through the oxide 102 from the source 101a of the substrate 101 to the floating gate 103. In the erasing mode, negative bias, for example, is introduced on the control gate 105 while positive bias is applied on the drain 101b to force the electron out of the floating gate to the source, as shown in FIG. 1b. 
In the prior art, a set of data can be programmed or erased at one time. The number of the memory units in the flash memory means the number of cells that can be programmed or erased. If there exist two sectors to store the data in a single cell, respectively, the device may program or erase two sets of data. Therefore, the capacity of the data set that is programmed or erased by this flash device is twice of the number of the traditional memory units in the flash memory.
The object of the present invention is to disclose a flash device with multi-bits cell capable of storing multi binary information bits. The further object of the present invention is to provide the method of forming the memory.
A method for manufacturing a nonvolatile memory capable of storing multi-bits binary information is disclosed. The method comprises the steps of forming an oxide on the semiconductor substrate. A conductive layer is formed on the oxide layer. A conductive layer is patterned to form a gate structure to act as a control gate. A first isolation layer is formed over the gate structure. A second isolation layer is formed over the first isolation layer. Then, performing an etching to etch the first isolation layer and the second isolation layer, thereby forming a L-shape structure attached on sidewall of the gate structure and a spacer on the L-shape structure, wherein the spacer functions as floating gate. Subsequently, an ion implantation is performed using the gate structure and the spacer as a mask to form source and drain regions in the semiconductor substrate adjacent to the spacer, wherein a channel under the gate structure keeps a distance from the source and drain regions. This distance connected between channel and drain region (or source region) is known as xe2x80x9cfringing field induced channelxe2x80x9d which is to be turned on by the gate voltage induced electric fringing field.
A silicide material is further formed on the gate structure and the source and drain regions. The silicide material includes TiSi2, CoSi2 or NiSi. Wherein the spacer is formed by an anisotropical etching and the first isolation layer includes SiO2 or HfO2. The second isolation layer includes nitride.
A nonvolatile memory capable of storing multi-bits binary information bits is provided. The memory includes an oxide formed on a substrate. A control gate is formed on the oxide. A L-shape structure is attached on sidewall of the control gate. Spacers are formed on the L-shape structure to act as a floating gate. A first doped region and a second doped region is formed in the substrate adjacent to the spacers. Wherein the spacers represent a first binary status by injecting and storing electrical charge in the spacers or to represent a second binary status by not injecting electrical charge into the spacer. A first and a second fringing field induced channels are under the first and the second spacers, wherein the first and the second fringing field induced channels are located between the main gate-induced channel. The first and the second doped regions sit adjacent to the first and the second fringing field induced channels, respectively. The main gate-induced channel is referred to the channel generated under the gate by conventional fashion. The fringing field induced channels are referred to the hot carrier injection channel under the spacers.
In the scheme according to the present invention called xe2x80x9cForward Program and Reverse Readxe2x80x9d, a high voltage is performed to program at one doped region side where charges are stored in the same side of spacer. Another high voltage is performed at the reverse side of doped region to read these charges. Furthermore, depending upon the geometry of the gate, the cell includes at least four sidewall spacers around a square gate structure. Under such arrangement, the present invention is capable of storing 4-bits information rather than two bits. A method of programming a nonvolatile memory capable of storing multi-bits binary information is disclosed. The memory cell includes a plurality of doped regions with a channel there between. It also includes a gate above the channel, a L-shape dielectric layer is formed on side wall of the gate. The spacers is attached on the L-shape dielectric layer, the method comprising: programming a first bit by applying programming voltages to the first doped region and to the gate and applying programming current or ground potential on the second doped region, thereby injecting and storing electrical charge in a first spacer close to the first doped region to represent the first binary status, or to represent a second binary status by not injecting electrical charge into the first spacer; programming a second bit by applying programming voltages to the second doped region, to the gate and applying the first doped region with programming current or ground potential, thereby injecting and storing electrical charge in a second spacer close to the second doped region, or by not injecting electrical charge into the second spacer; programming a third bit by applying programming voltages to the third doped region of the plurality of doped regions and to the gate while applying the fourth doped region with programming current or ground potential, thereby injecting and storing electrical charge in a third spacer adjacent to the third doped region, or by not injecting electrical charge into the third spacer.
The programming may further comprises programming a fourth bit by applying programming voltages to the fourth doped region and applying the same bias to the gate while applying the third doped region with programming current or ground potential, thereby injecting and storing electrical charge in a fourth spacer adjacent to the fourth doped region, or by not injecting electrical charge into the fourth spacer.
A method of programming a nonvolatile memory capable of storing multi-bits binary information bits, the memory cell having a first doped region and a second doped region with a channel therebetween and having a gate above the channel, a L-shape dielectric layer formed on side wall of the gate, spacers attached on the L-shape dielectric layer, the method comprising: erasing a first bit of the two binary information bits by applying erasing voltages to the gate and a first doped region such that to cause charge representing a first binary status to be removed from a first spacer of the spacers for charge trapping; erasing a second bit of the two binary information bits by applying erasing voltages to the gate and a second doped region such that to cause charge representing a second binary status to be removed from a second spacer of the spacers for charge trapping.
A method of reading an nonvolatile memory capable of storing multi-bits binary information bits, the memory cell having a first doped region and a second doped region with a channel therebetween and having a gate above the channel, a L-shape dielectric layer formed on side wall of the gate, spacers attached on the L-shape dielectric layer, the method comprising: applying a reading bias on the gate and a second doped region, the reading bias having levels lower than the voltages applied during programming for sensing a channel current to determine whether the channel current is significant representing a first binary status in the spacer adjacent to the first doped region or the channel current is relative low to the significant channel current representing a second binary status in the spacer adjacent to the first doped region; applying the reading bias on the gate and a first doped region, the reading bias having levels lower than the voltages applied during programming for sensing the channel current to determine whether the channel current is significant representing the first binary status in the spacer adjacent to the second doped region or the channel current is relative low to the significant channel current repressing the second binary status in the spacer adjacent to the second doped region.